Method of bleaching



Patented July 1, 1930 UNITED STATES PATENT OFFICE LINN BRADLEY, OFMONTCLAIR, NEW JERSEY, AND EDWARD P. MGKEEFE, OF PLATTS- BURG, NEW YORK,ASSIGNORS TO BRADLEY-MCKEEFE CORPORATION, OF NEW YORK, N. Y.,. A.CORPORATION OF NEW YORK METHOD or BLEAGHING No Drawing.

This invention relates to improvements in the bleaching and purificationof fibers and fabrics or other materials, especially cellulose from woodfor use in the manufacture of pulp and paper products, etc. In thecommon 'methods of bleaching pulp with chlorine -or chlorine-bearingcompounds, such as bleaching powder, the treatment tends to injure orweaken it. Wood pulp bleached with chlorine compounds frequently has anatural wood tint rendering it inferior for the better qualities ofproduct and its ash content may be greatly increased by the bleachingoperation particularly 1 when bleaching powder is used.

The improved bleaching process of the present invention is amultiple-stage process in which-the pulp is subjected-first to theaction of a suitable oxidizing bleaching agent present in an amountinsufiicient to complete the bleaching treatment, and in which thebleaching operation is completed by a subsequent treatment withbleaching materi al. Each stage may employ the same and/or differentbleaching reagents and any or all of the stages be either acid oralkaline, according to. the reagents chosen and the results desired.

According to the present invention. we

avoid some of the usual objections to bleaching with chlorine orchlorine-bearing compounds, and we produce a high white celluloseproduct, low in ash, and of good strength, thus making it suitable for.many purposes for which cellulose bleached by former methods has beenfound less satis factory.

According to the present invention one or more stages of the bleachingoperation may be carried out with a manganate or permanganate asdescribed in our co-pending application Serial No. 449,245, filed March2, 1921, of which this application is a continuation in part. Or theinvention may be practiced by the use of bleaching solutions containingan active chlorine-bearing reagent, as exemplified by our co-pendingapplication Serial No. 544,322, of which this application is acontinuation in part.

ccording to one embodiment of the pres- Application filed March 14,1928. Serial No. 261,697.

ent invention, we may employ a manga: nate or permanganate inthebleaching operation. When a solution of a manganate or permanganateis added to the material to be bleached or oxidized, not only does themanganate or permanganate exert an oxidizing action on the bleachable oroxidizable constituents of the material, but these constituents exert areducing action which results in the precipitation of a compound orcompounds of manganese, so that the material is given a brown color.This production of a compound of manganese takes place not only when themanganate or permanganate is alkaline, but even when sufficient acid isadded to give a mildly acid reaction, unless a relatively large excessof acid is used or the material is heated to a high temperature orallowed to stand a prolonged .time with a moderate amount of acid. Thecompound of manganese thus precipitated is to a considerable extentdeposited directly on the fibers,

and this production of a compound of manganese appears to take placecoincidently with the action of the manganate or permanganate upon thematerials associated with the fibers; although the formation of acompound of manganese may take place in the solution where the solutioncontains 0x1- dizable constituents.

After the treatment of the material to be bleached with the manganate orpermanganate (if a sutliciently large excess of acid and hightemperatures have not been used ,or the mix allowed to stand for a longtime while moderately acid) the material is, as above stated, of a browncolor. It is now treated to dissolve away or remove a manganesecompound, preferably by the addition of a sufficient amount of sulfurdioxide or a solution of sulful dioxide, or of a bi-sulfite, or asulfite and anacid, or any other suitable reagent. When sulfur dioxideor other suitable reagent is added to the material to be thus treated,the brown color promptly disappears and the material is left white. Themanganese: compound is thus rendered soluble and can be washed free fromthe bleached material.

We have found that manganates can be successfully used for the bleachingprocess of the present invention by using an amount larger thanwhenpermanganates are used and we include the use of manganates as wellas permanganates. However, their bleaching effect is less efficient thanthat of permanganates; and we regard the use of permanganates as moreadvantageous. The permanganates can be produced from manganates byoxidation with suitable agents; and this production of permanganates andtheir subsequent use according to the present invention canadvantageously be combined. For example, sodium manganate, admixed withan excess of alkali, may be treated in solution with chlorine to oxidizesodium manganate to sodium permanganate and to form sodium hypochloritewith excess alkali; and the resulting mixture can then be utilized as acomposite bleaching or oxidizing agent, as hereinafter more fullydescribed.

According to the present invention, instead of adding the bleachingliquor, e. g., including manganate or permanganate, all at one time insufficient amount to complete the desired bleaching treatment, we havefound that an economy in the required amount of bleaching material canbe effected by carrying out the bleaching in stages, i. e., by addingpart of the bleaching liquor and permitting it to act upon the fibrousmaterial, then removing soluble compounds (resulting from this partialbleaching) by removing the spent liquor from the fibrous materialwashing etc., and then adding to the partially bleached fibrous materialsome of the same or a different kind of bleaching liquor. Two or moresimilar treatments, as desired, may be employed until the requiredbleaching result is attained. When onlya small or moderate amount ofbleaching material is used in this way, i. e., in the first orpreliminary partial bleaching treatment, there appear to be producedcompounds which are water-soluble and may be substantially colorless,some of which are apparently not fully oxidized, i. e., are capable ofconsuming permanganate in becoming oxidized to a further degree. We havefound that these compounds can be removed from the partially-bleachedfibrous material by the washing or liquor removal operation, orotherwise, so that they will not be present 1n a subsequent stage toconsume bleaching material, such as for example, a manganate orpermanganate.

The amount of sulfur dioxide or other sulfurous material required forconverting the brown material, if any produced, by

. action of amanganate or permanganate,

may vary somewhat with the amount of manganate or permanganate consumed.Ordinarily only a small or moderate amount is required to form a solublemanganese compound and bring the material to a white color, provided asufficient amount of bleaching material has been consumed and thematerial previously washed, but sufficient sulfur dioxide should be usedto insure dissolving all the undissolved manganese compound remainingupon the fibers.

It is one advantage of the present invention that the bleachingmaterial, when an alkali metal compound is used, is sufficiently solubleso that the finished bleachedand washed fibrous material is low in ashas well as of good white color and excellent texture. The bleachingprocess, moreover, is one which can be carried out at a moderatetemperature.

As above stated, the permanganate which may be used in the presentinvention, may advantageously be produced from manganates. Alkali metalmanganates are commonly made by fusion methods which leave an excess ofalkali, such as sodium hydroxide, with the manganate, when economicyields are desired. Instead of neutralizing this excess of alkali withan acid, or otherwise removing it, we have found that it is advantageousto leave it with the manganate, and then treat the mixture of manganateand excess alkali in aqueous solution, or in suspension in part, withchlorine, either after or before adding the mixture to the fibrousmaterial to be bleached. Chlorine will act upon the manganate to formpermanganate and it will also act upon the excess alkali to formhypochlorite, so that if a proper amount of chlorine be added there willbe produced a composite solution containing both. permanganate andhypochlorite. This composite solution may be used in one or more stagesfor bleaching the pulp. in the multi-stage process of the presentinvention, or it maybe used in only one of the stages, and a differentsolution, such, for example, as a solution containing a chlorine-bearingbleaching reagent, may be used in another stage. The hypochlorite insuch composite bleaching liquorappears to act in a different and moreadvantageous manner than when used alone, while the action of thepermanganate is supplemented by the action of the hypochlorite, so thata lesser amount of permanganate need be used thanwhen permanganate aloneis relied upon for obtaining a similar degree of bleaching.

The composite bleaching liquor may be directly made in solution at thetime and place of use. For example, if sodium manganate is made by afusion method and contains an excess. of alkali, this can be shipped ina solid state to a laundry, textile factory, paper mill, etc., dissolvedin water, and the solution treated with a suitable amount of gaseouschlorine sufficient to oxidize the manganate and produce hypochloritefrom substantially all available alkali, and this solution (afterfiltering if necessary) can 'in hypochlorite liquor, etc.

then be added directly to the pulp or other material to be bleached. Ifthe manganate or permanganate contains an insuflicient amount of excessalkali to produce the desired amount of hypochlorite, suitable alkali oralkaline compounds can be added before the treatment with chlorine, andthe com-.

posite solution will be obtained.

Instead of producing the composite solution containing permanganate andhypochlorite and adding this solution to the pulp or other materials tobe bleached, the manganate or permanganate and excess alkali or othersuitable compounds can be added to and mixed with the pulp or othermaterial to'be bleached and the proper amount of chlorine thenintroduced, for example, in the form of a chlorine solution orpreferably in gaseous form, care being taken to secure good distributionand absorption of the chlorine.

The proportions of permanganate and hypochlorite in the composite bleachsolution can be varied and regulated, for example, by varying andregulating. the amount and proportion of free alkali in admixture withthe manganate which is to be treated with chlorine. So also, a compositebleach solution of regulated permanganate and hypochlorite content canbe made by admixing separately produced permanganate and bypochloritesolutions 0 known permanganate and hyprochlorite content, or the desiredamount of permanganate may be dissolved Such solutions can be producedand admixed prior to the addition of the composite bleaching solution tothe pulp or other material to be bleached, or the mixture may takeplace, for example,

- in the pulp by separately adding the solutions thereto, withappropriate agitation of the pulp to bring about uniformity of dis-'tribution of the reacting materials. The regulation of the proportionsof permanganate and hypochlorite makes possible the regulation of thebleaching operation ina simple and advantageous manner, as will bereadily appreciated.

When the pulp or other material 'to be bleached has been treated withthe composite bleaching agent, it can be subsequently treated withsulfur dioxide solution or other suitable reagent in the manner abovedescribed. By washing the'pulp or other material to be bleached, afterthe manganate or permanganate and hypochlorite treatment, the pulp,etc., can be freed from the greater part of the compounds in solution,prior to the sulfur dioxide solutionorother treatment for-dissolving amanganese compound.

. Instead of using sulfur dioxide for converting oxide or othercompounds of manganese into a soluble form, other agents which con- Vertthe oxlde into colorless and soluble niaperiod of time, etc. the time ofthe cooking operation makes terial can be similarly used. Acids alonecan be used for this purpose, such as hydrochloric acid or sulfuricacid, but a consider able amount of such acid will be required unlessthe action is carried out or finished at an elevated temperature, orallowed to stand for considerable time and the fibers may be therebyinjured. The treatment with sulfur dioxide or sulfurous acid has theadvantage that it takes place readily and quickly at ordinarytemperature.

We have found that pulps obtained, for example, by the soda, sulfate andsulfite processes, so-called, are not only given; a white color, butretain their white color much better and longer when bleached accordingto the present invention with a manganate or a permanganate, eitheralone or assisted by chlorine or a hypochlorite, than when bleached bychlorine or a hypochlorite in single stage, according to the commonmethods of bleaching, i. e., without the use of a manganate orpermanganate. This advantage of increased permanence of whiteness of thebleached pulp is a characteristic advantage of the present invent-ion,as compared to pulps bleached by bleaching powder in the customarysingle-stage manner.

It is a further advantage of the present invention that it makespossible the satisfactory bleaching of chemical pulp from wood producedby what is commonly known as the short-time or quick-cook method, thatis, by carrying out the cook with cooking liquors of various degrees ofstrength or of concentration, pressure, temperature and time, andterminating the cook without carrying it to completion. Such pulp,produced by the short-time or qulck-cook method, so-called, cannot bereadily and satisfactorily bleached by ordinary bleaching methods, andusually results in a heretofore would be considered under-- cooked anddifiicult to bleach by common bleaching methods can advantageously bebleached by the process of the present invention and brought to a goodwhite color with materially less injury to the stock than with methodsof bleaching heretofore commonly used. The present invention makespossible therefore the production of a satisfactory bleached pulp fromunder-cooked stock, thus avoiding the necessity in many cases ofcontinuing the cooking operation for the normal The cutting down ofpossible a material increase in the number of digestions which can becarried out in a given apparatus, and correspondingly increases theproducing capacity of the digesters over the usual practice in the art.

We accordingly include and claim as a part of our invention theproduction and bleaching of such under-cooked pulp by'treating suchpulp' in the manner herein described.

111 addition to the bleaching of under-.

cooked pulp, the process of the present invention is also of particularvalue for bleaching pulps which are difficult to bleach by commonbleaching methods, and enables a relatively superior bleached pulp to beobtained thereby.

Thereare large supplies of forest products, etc., which, due to theirinherent composition and properties, have been unsuited to theproduction of high grade cellulose material by the processes heretoforecommonly practiced. We have found that many such materials canadvantageously be treated for the production of pulp or cellulosematerials by the use of cooking liquor containing an alkali metalsulfite, as more fully described in companion applications. The

' bleaching process of the present invention is of special value inconjunction with such a cooking process, and gives a high grade whitepulp from cellulose material. In this way certain forest products,bagasse, reeds, grasses, bamboo, corn stalks, straws, etc., which arenot commonly considered suitable for production of high grade pulp orcellulose material, can be converted into white products of relativelyhigh grade. We, therefore, have increased the available supply of rawmaterials available for production ofhigh grade pulp or celluloseproducts. Even when such products are subjected to the usual methods ofcooking, the pulp obtained can be satisfactorily subjected to thebleaching process of the present invention to give a bleached pulp ofhigher grade and whiter in color than is obtained with the usualbleaching processes, i. e., employing bleaching powder in the customarymanner and amounts.

The process of the present invention, as

L above stated, is of more or less general application to the bleachingof fibrous material and fabrics, etc., of a bleachable character andwillgive whiter bleached materials and with less injury than with any ofthe usual bleaching methods of. which we are aware.

"While we have described the invention more particularly in connectionwith the use of alkali metal and alkaline earth metal manganates andpermanganates, the invention is not limited thereto, but other suitablemanganates and permanganates can be used which are capable of exerting ableaching or oxidizing action, and such manganates or permanganates canbe used either alone, as hereinbefore described, or in conjunction withother agents, e. g., hypochlorites, etc.

The invention will be further illustrated by the following more detailedexamples, but it will be understood that the invention is not limitedthereto. WVhen using a liquor containing manganate or permanganatealone, the treatment will vary somewhat, depending upon various factors,such as the kind and amount of organiomaterial to be oxidized, the kindof manganate or permanganate used, etc. Some chemical pulps derived fromwoods may require the use of from about'one to about two percent sodiumpermanganate or its equivalent, based on the air-dry weight of. the pulp(i. e., pulp containing ten percent water) and the liquor maybe heatedto around 30 to 40 C., more or less, until all or nearly all of thepermanganate has been consumed in oxidizing organic matter. Theresultant liquor, which may still contain some oxidizable organic matterin solution, is removed from the fibrous material and some suitableamount of manganate or permanganate solution is added to the partiallybleached fibrous material. An improved result is obtained by suchmultiple-stage process and the amount of permanganate required foreffecting the desired-degree of bleaching will ordinarily be somewhatless than when all of the permanganate is employed in one stage. Thusabout one-half the entire amount of permanganate used ordinarily may beadded to the unbleached fibrous material in the first stage, or lessthan one-half of such amount may be employed in the first stage, whilein some cases somewhat more than one-half of such amount may be employedin the first stage. After the final oxidizing treatment, the fibrousmaterial may be subjected to a treatment to remove manganese-bearingmaterial, as indicated above. In some cases the bleaching liquor used inthe oxidizing treatment may advantageously be slightly or moderatelyacid, but, if preferred, a neutral or moderately alkaline liquor may beused for the oxidizing treatment. life have found that it isadvantageous, when employing bleaching liquors capable of oxidizingnon-fibrous organic matter associated with fibrous material derived fromwood, for example, chemical pulp, to so conduct the first or preliminarybleaching stage that all or substantially all of the oxidizing power ofthe bleaching agent or agents added in regulated amount (less thansuflicient for the entire amount of oxidation to be effected) is"consumed before the resulting liquor and its dissolved constituents areremoved from the resulting fibrous material, and further, so that suchliquor when removed from such partially bleached material (fibrous),contains organic matter which is capable of reacting with oxygen derivedfrom a permanganate such as potassium permanganate. For example, bytreating such unbleached chemical pulp with chlorine alone, or with ahypochlorite alone, or with a manganate or permanganate alone, or withvarious of these reagents simultaneously or in conjunction or in anyother manner, but with the amount of such reagent or reagents employedin the first stage or preliminary bleaching treatment being less, suchas for example, one-half, more or less, than the entire amount ofbleaching agent or agents needed for the desired total degree ofbleaching, and consuming all or substantially all of such reagent orreagents (supplied in the first stage) in reaction or.

reactions with organic matter thereof, the resulting liquor will in mostcases, if not all, contain some organic matter which is capable ofreacting with oxygen from a dissolved oxidizing agent such as potassiumpermanganate. By removing such a liquor from the partially bleachedfibrous material, and particularly if the fibrous material is washedbefore the second treatment, such oxidizable organic matter is removedfrom the fibrous material before the second or subsequent addition ofbleaching agent or agents is made thereto. Thus we have made it possibleto reduce the amount of bleaching agent or agents required for thecomplete bleaching process.

As another example of how our invention may be practiced, chlorine, orhypochlorite may be used to bring about a partial bleaching of suchfibrous material and the oxidizing bleaching treatment can then becompleted with a'manganate or permanganate.

Instead of using a permanganate solution, bleaching solutions can beused containing chlorine-bearing compounds alone or in combinationwithother reagents.

A pulp may be prepared from jack-pine or loblolly-pine, or pines of asimilar character, by cooking the pinewood chips with a cooking liquorcontaining a preponderating amount of caustic soda and a lesser amountof sodium sulfide, for example, 15 parts by weight of caustic soda and 5parts by weight of sodium sulfide per 100 parts by weight of wood(figured on the air-dry basis) at a temperature corresponding to asaturated steam pressure of 110 pounds for a period of about 6 hours.The resulting pulp may be freed from the residual liquor and washed inthe usual way. It may be then .diluted with sufficient water to enableit tobe readily handled (e. g., containing 1 to 5 of fibers) and thentreated with chlorine by admixing a hypochlorite solution (e. g.,bleaching powder) therewith and liberating the chlorine or hypochlorousacid in place by means of added mineral acid, such as sulfuric acid.Instead of producing the chlorine or hypochlorous acid in this way,chlorine gas can bedirectly introduced into and absorbed by the pulp.Only a moderate amount of chlorine is necessary, for example, 3 to 15pounds, more or less, of bleaching powder per 100 pounds of pulp(air-dry basis). Sufiicient mineral acid is thereafter admixed to givean acid reaction. This chlorine treatment can be carried out at ordinaryatmospheric temperature and requires but a few minutes for itscompletion, ordinary care being taken to uniformly treat the pulp andutilize the chlorine upon the fibrous material in an economical manner.

After such a preliminary acid bleaching treatment, the excess liquor maybe removed, for example, by means of a suitable pulp thickening device,either with or without previous addition of soda or lime to fix any freeacid, and the pulp is preferably washed. with water to remove ,aconsiderable part of the dissolved organic matter that may be presentwith the pulp. The pulp is then subjected to a further bleachingoperation with an alkaline hypochlorite bleach, e. g., chloride of limeor bleaching powder, although the amount of such bleaching agent can besomewhat decreased from that commonly used, when a single bleachingtreatment with this reagent is relied upon. The amount of hypochloritesolution, may, for example, be the equivalent of from 3 to 15 pounds ofbleaching powder per 100 pounds of pulp (air-dry basis), and thebleaching can be promoted by heating the pulp to a moderate degree. Whenthe bleaching operation is thus completed, the pulp is washed to free itfrom the residual solution, and is then ready for use as a bleachedpulp.

The bleaching of such pine pulp in the manner. above described has beenfound to give a good strong white pulp which holds its colorsatisfactorily even upon prolonged exposure to air. When the pulp ismade with a cooking liquor, such as a cooking liquor containing apreponderating amount of normal sodium sulfite with a moderate amount ofother digesting material, such as sodium sulfide or caustic soda, whichis without seriously objectionable action upon the fibers, the resultingpulp will contain fibers of a substantially uninjured condition suchthat the pulp will contain strong fibers which maybe thus bleached to asatisfactory white color, without undue injury.

The preliminary treatment of the pulp with chlorine can be effected byintroducing chlorine or chlorinated water or an acid hypochloritesolution into the pulp or producing such compound therein, e. g., on 1tsway to a thickening device preferably of acid-rethe thickening and theremoval of the residual bleach liquor takes place at the same time. Thepreliminary acid bleaching operation appears to alter or remove certainresistant organic matter or otherwise dissolve or change certain of theconstituents present so that the subsequent bleaching, for example, withchloride of lime, can be readily carried out with possible saving intime and in steam for heating, while giving an improved bleached pulpwhich has not been subjected to the injurious action of a strongeralkaline bleach liquor at high tempera tures. The bleached pulpaccordingly will have an improved color as well as improved propertiesin other respects, and the bleached pulp obtained from pines will bedifferent from such pulps bleached by the usual method.

According to another application of the invention, the preliminarybleaching operation can be carried out by adding chloride of limesolution or sodium hypochlorite solution to the unbleached pulp and alsoadding a limited and regulated amount of acid sufficient to give thepulp an acid reaction and to liberate chlorine or hypochlorous acid,and, after the first bleaching treatment, a predetermined amount ofalkali can be added sufiicient to fix the free acid and enoughadditional alkaline hypochlorite (e. g., bleaching powder or sodiumhypochlorite solution) may be added to finish the bleaching operation,the pulp being moderately heated during the second bleaching.

treatment which is advantageously carried out under non-acid conditions.

We consider it more advantageus, however, to subject the pulp to awashing operation after the preliminary acid bleaching treatment andbefore the subsequent bleaching treatment, in order to remove certainorganic matter from the pulp and free it from any remaining free acid,and from compounds rendered soluble by the acid bleaching treatment,before adding alkaline hypochlorite for the second bleaching treatment.

Pulp which is diflicult to bleach with an alkaline hypochlorite solutionalone (e. g., a solution, of bleaching powder) can nevertheless bereadily bleached by the improved multi-stage treatment of the presentinvention. For example, we have found that Kraft pulp, produced fromspruce by the.

sulfate process so-called and which was diflicult to bleach withbleaching powder alone, could be readily bleached to a strong white pulpby the present process. \Ve have also found that pulp from Canadajack-pine, Minnesota jack-pine, Virginia loblolly-pine,

yellow pine, etc., (produced by means of a cooking liquor made upessentially of a limited amount ofcaustic soda and a lesser amount ofsodium sulfite) could be readily bleached by our new method to abeautiful strong bleached pulp with a moderate amount of bleachingagent.

The second stage of the multi-stage bleaching processican be carried outat ordinary temperature if a somewhat increased time is permitted forthe bleaching operation, especially with pulp such as poplar pulp madeby the soda process and spruce pulp made by the so-ca'lled (acid)sulfite process, although it is better to warm the liquor to a moderatedegree. Bleached pulps of excellent white color can be thus obtainedwith relatively decreased injury to the pulp in that the process avoidsthe severe treat ment with alkaline hypochlorite at an excesstemperature such as tends to oxidize the pulp with resulting productionof oxycellulose or to modify the pulp with resulting production of highamounts of beta and gamma cellulose, etc. The second stage of thebleaching process can be carried out by the addition of sodiumperborate, sodium peroxide or by permanganates, for example, by sodiumor potassium permanganate, using either an alkaline permanganatesolution or one which is made acid by the addition of a suitable acid insufficient amount. When an alkaline permanganate solution is employed,the pulp preferably should be warmed somewhat to facilitate or expeditethe bleaching operation. When an acid solution is used, the bleachingtakes place readily at ordinary temperatures although a moderate heatingmay be employed. The manganese compound deposited on the fibers can bereadily removed by a solution supplied with sodiunr sulfite or bisulfiteand 'sulfuric acid, or by a solution of sulfurous acid, and the pulp canthen be freed from dissolved reagents by washing.

The following example shows another application of the invention;

Short leaf pinc was cooked with cooking liquor containing caustic sodaand sodium sulfite for a period of about four hours, the amount ofcaustic soda being about 15 parts by weight and the amount of the sodiumsulfite 25 parts by Weight in 450 parts of liquor used for treating 90parts by weight of wood (calculated on a. bone-dry basis). The pulpobtained was of excellent quality, with long and strong fibers, but itdid not bleach readily to a white pulp in a single stage with a calciumhypochlorite solution. It was readily bleached, however, when treatedwith chlorine in the presence of Water at ordinary room temperature forabout 15 minutes, then washed and treated with calcium hypochlorite(bleaching powder) solution and moderately warmed. The resulting pulpwas a good strong white pulp.

Cotton linters, black gum, red gum, tupelo gum, chestnut, white oak,black oak, red oak, hickory, birch, beech, maple, bamboo,

jute and other pulps have been bleached by chlorite bleaching liquor, inwhich the acid bleaching liquor is preferably and advantageouslyremoved, for the most part, from the pulp, before the second stage ofthe process. The preliminary stage can be carried out with the use ofchlorine gas added to the pulp or by the use of.added bleaching powderor sodium hypochlorite and with added acid such as sulfuric acid to givean acid reaction. The second bleaching operation can then be carried outat ordinary temperature for sufiicient time or with moderate warmingwith a solution of calcium hypochlorite orbleaching powder or sodiumhypochlorite. The second bleaching operation can also be carried outwith sodium perborate, sodium peroxide or with sodium or potassiumpermanganate and with or without sulfuric acid with subsequent treatmentof the pulp with sodium bisulfi'te' and sulfuric acid, or a solution ofsulfurous acid, followed-by a washing operation.

Instead of using an active chlorine reagent alone or an oxidizingmanganese compound alone, a composite liquor may be used containing bothof these oxidizing agents, for example, a liquor may be preparedcontaining from about one-fourth part to about three parts, more orless, of sodium permanganate, or its chemical equivalent, and aboutone-fourth to fifteen or twenty parts, more or less, of sodiumhypochlorite or its chemical equivalent, such amounts of bleachingreagents being based on the air-dry weight of the pulp, such aschemicalwood pulp. Suitable ratios of such reagents within the limits named maybe chosen as required to effect the desired degree of bleaching and aportion of the liquor, ordinarily about one-half, will be added to thepulp for the first treatment and the remaining portion added in oneormoresubsequent treatments after the removal of spent liquor from theresulting fibrous material.

In many cases, as for example where the pulp has been produced by analkaline cooking treatment suclf' as the so-called sulfate process, orwhere permanganate is used, it isadvantageous to use a bleaching liquorwhich isslightly acid. However, since an acid reacting liquor requiresthe use of special acid-resisting apparatus, such advantages may beoverridden by other practical considerations and our process is equallyapplicable in cases where alkaline liquors are used in one or all-of thesteps, as where acid liquors are used in any or all.

In such cases an alkaline manganate or permanganate solution may beusedin both stages, or composite liquors as describedabove may be used inone or both stages, or the alkaline liquor in one or both stages may bea solution of an alkaline hypochlorite.

The following is an example of how the two-stage process may beperformed using an alkaline hypochlorite bleaching solution. Thewell-washed and unbleached pulp, e. g., spruce pulp obtained by theconventional acid-sulfite process, composed of around three to sevenpounds of fiber to around 97 to'93 parts of water, more or less, may betreated with a bleaching powder liquor prepared by dissolving calciumhypochlorite in cool water and such bleaching liquor may have a smallamount of calcium hydroxide in it so as to render it moderatelyalkaline. The amount of bleaching liquor added to the pulp for thefirst-stage treatment may vary somewhat. Thus an amount of bleachingliquor which contains around five to ten 7 parts, more or less, ofbleaching powder (35% available chlorine) for each 100 parts, by weight,of fibers, may be added to the pulp and thoroughly mixed therewith; thenthe mixture mav be gradually heated while it is being well agitated andcirculated, by

the addition of steam in regulated amount,

for example, such as will not overheat one portion to an objectionabletemperature, and the temperature of the mixture gradually brought up ina substantially uniform manner to around 30 to 60 degrees centigrade,more or less.

The treatment may be thus continued until the liquor shows by test thatsubstantially all of its available bleach has been exhausted; then themixture may be subjected to a dewatering operation by means of anysuitable device, and the resulting pulp may with advantage be given awashing treatment by means of water so as to remove a large amount orsubstantially all of the bleach residues from the pulp. Then thedewatered and preferably washed pulp is admixed with an additionalcharge of bleaching liquor of the kind described above, the amount thusadded being s'ufficient or somewhat more than sufficient to carry thebleaching to the desired degree. The mixture of pulp and added bleachingliquor is circulated and gradually heated and thoroughly mixed, theheating being continued until a temperature of around 30 to 60 0., moreor less, has been reached.

hen the fiber has been bleached to the desired degree, the bleachresidue is removed in anysuitable manner and the bleached fiber thenwell washed with water.

The amount of bleaching liquor added for the second-stage treatment mayvary somewhat, depending upon the material to be bleached, the degree ofwhiteness desired, the amount of bleaching reagent utilized in thefirst-stage treatment, etc. Generally,

an amount of bleaching liquor carrying from five to ten parts ofbleaching powder (35% availablechlorine) is sufiicient for thesecond-stage treatment; and the larger the amount of bleach used on thefirst-stage treatment, the lower the amount required for thesecond-stage treatment.

In case the bleach residue removed from the secondstage treatment in anyof the above examples, carries a substantial or rather large amount ofunconsumed bleach ing reagent, which can be determined by suitabletests, such residue can be utilized, at least in part, for a preliminarybleaching treatment of the unbleached pulp, and a regulated additionalamount of bleaching material may be added thereto so that the bleachingliquor for the first-stage treatment may be composed (in part) of bleachresidue from the second-stage treatment and (in part) of freshlysupplied bleaching material. Thus a fair or moderate excess of bleachingmaterial, over that actually required for the secondstage bleachingtreatment, may be added in the second-stage, and

then when the color of the pulp has come up to the desired point, thesurplus may be removed along with the liquor removed from the bleachedpulp and later employed in apreliminary bleaching treatment of unbleached fibrous material. When operating in the latter manner, careshould be taken to avoid heating the liquor in the secondstage to such atemperature that an objectionable amount of available chlorine is lostfrom the liquor owing to such heating treatment.

In the accompanying claims we have used the term a hypochlorite of analkali-forming metal as including hypochlorites of alkali metals such assodium hypochlorite and the hypochlorites of alkaline earth metals suchas calcium. hypochlorite or chloride of lime.

e claim:

1. The method of bleaching fibrous material which comprises addingthereto an oxidizing bleaching liquor in an amount insufficient toproduce the complete bleaching result, permitting such liquor to remainin contact with the fibrous material until its bleaching action issubstantially exhausted, removing spent liquor and soluble compound fromthe fibrous material and subsequently completing the bleaching to the desired degree by means of another addition of oxidizing bleaching liquor.

2. The method of bleaching fibrous material which comprises addingthereto a bleaching liquor containing oxidizing material in an amountinsuflicient to produce the complete bleaching result, permitting suchliquor to remain in contact with the fibrous material until itsbleaching action is substantially exhausted, removing spent terial inexcess of the amount required to.

complete the desired bleaching result, separating liquor from thebleached fibrous material and utilizing unconsumed bleaching material inthe liquor for treating unbleached fibrous material. a

3. The method of obtaining fibrous material suitable for being convertedinto white paper, which comprises digesting fibrous material, by meansof liquor including a sulfite of an alkali, separating residual liquorfrom the fibrous material, washing the fibrous material and subjectingthe washed fibrous material to the action of a bleaching liquorincluding an oxidized compound of manganese, said liquor being added insuccessive portions and spent liquor being removed between suchadditions, and thereafter treating the resulting fibrous material toremove colored compounds of manganese and to obtain a white fibrousmaterial.

a. The method of bleaching cellulosic fibrous material, which comprisessubjecting such fibrous material to the action of a reagent capable ofexerting an oxidizing bleaching treatment upon non-fibrous organicmaterial associated with the fibers, in the presence of water, removingresulting liquor from the resulting fibrous material, and thereaftersubjecting the resulting fibrous material to the action of a reagentcapable of exerting an oxidizing bleaching treatment upon non-fibrousorganic material associated with the fibers, one of such reagents beinga reactive oxygen-bearing compound of manganese.

5. The method of bleaching cellulosic material, which comprises addingthereto such an amount of bleaching material including an oxygen-bearingcompound of manganese,

' capable of exerting a bleaching action, as is required for bleachingthe cellulosic material, in a series of additions, and removing, betweensuch additions, soluble compounds from the fibrous material after a partof such bleaching material has exerted its ac tion and before the lastaddition of such bleaching material is made.

6. The method of bleaching fibrous material derived from wood, whichcomprises adding thereto such an amount of bleaching material includingan oxygen-bearing-compound of manganese, capable of exerting a bleachinaction, as is required for bleaching the brous material, in a series oftwo or more additions, and removing, between such additions, solublecompounds from. the fibrous material after a part of such bleachingmaterial has exerted its action ant before the last addition of suchbleaching material and subsequently treating the fibrous material toremove a water insoluble compound or compounds of manganese.

7. The method of bleaching undercooked wood pulp, which comprises addingthereto such an amount of bleaching material including an oxygen-bearingcompound of manganese, capable of exerting a bleaching action, as isrequired for bleaching the fibrous material, in a series of two or moreadditions, and removing, between such addie tions, soluble compoundsfrom the resulting fibrous material after a part of such bleachingmaterial has exerted its action and before the last addition of suchbleaching material. I

8. The method of bleaching cellulosic fibrous material, which comprisessubjecting the fibrous material to the action of liquor containing amanganate of an alkali metal in amount less than sufficient to completethe bleaching treatment, removing soluble oxidizable compounds from thefibrous material, and subsequently subjecting the fibrous material to ableaching treatment under conditions adapted to bleach remaining organicmatter.

9. The method of bleaching cellulosic fibrous material, which comprisessubjecting the fibrous material to the action of liquor containingsodium permanganate in an amount less than sufiicient to complete thebleaching treatment, removing resulting liquor and soluble oxidizablecompounds from the fibrous material, subsequently treating the fibrousmaterial with sodium permanganate, and subsequently treating the fibrousmaterial to remove a manganese compound therefrom.

10. The method of bleaching fibrous material derived from wood by meansof a digesting treatment, which comprises treating such material bymeans of oxidizing material in such amount and under such conditions asare adapted to effect at least onehalf of the desired total bleaching,removing soluble material from resulting fibrous material, andsubsequently subjecting such re sulting fibrous material to. anoxidizing treatment adapted to complete the bleaching.

11. The method of bleaching cellulosic fibre-bearing material, whichcomprises treating such material by means of oxidizing bleachingmaterial in such amount and under such conditions as are adapted toeffeet a partial but incomplete bleaching thereof and to produce aliquor which contains dissolved matter capable of reacting withavailable oxygen contained in sodium permanganate, removing suchdissolved matter from resulting partially bleached fibrous material, andsubstantially subjecting such resulting fibrous material to an oxidizingtreatment adapted to complete the bleaching.

12. The process according to claim 11, in

which a chlorine-bearing compound is employed in at least one of thebleaching treatments.

13. The process according to claim 11, in which a chlorine-bearingcompound, capable of exerting an oxidizing treatment upon organic matterpresent in unbleached chemical pulp derived from wood by means of adigesting treatment, is employed in at least two oxidizing treatments.

14. The multiple-stage process of bleaching chemical wood pulp whichcomprises the following steps: (a) mixing the unbleached fibrousmaterial with a liquor containing an oxidizing bleaching material; (6)treating the mixture so as to react a portion of the non-cellulosicorganic material with bleaching material comprising a bleaching reagentderived from the bleaching material supplied in step (a) (c) removingresidual liquor containing soluble reaction products from thispartially-bleached fibrous material; ((Z) admixing with this partiallybleached fibrous material additional bleaching liquor including anoxidizing bleaching reagent and thereafter treating the fibers so as toconvert them into bleached chemical pulp, the amount of availablebleaching ma terial supplied to the fibrous material in thefirst-mentioned mixture being less than Would be sufficient to convertthe unbleached pulp into high white bleached chemical pulp in asingle-stage bleaching treatment.

15. The process according to claim 14, in which the total amount ofavailable bleaching material supplied to the fibrous material is lessthan would be sufficient to convert the unbleached pulp into high whitebleached chemical pulp in a single-stage bleaching treatment.

16. The process according to claim 14, in which at least one-half of thetotal amount of available bleaching material supplied to the fibrousmaterial is supplied to the fibrous material prior to the step a.

17. The process according to claim 14, in which substantially the totalamount of available bleaching material supplied to the fibrous materialprior to step 0 is consumed in the bleaching treatment prior to step 03.

18. The multiple-stage process of bleach ing chemical wood pulp, whichcomprises the following steps: (a) mixing the unbleached fibrousmaterial with bleach liquor including an oxidizing'bleaching material;(6) treating the mixture so as to react a portion of the non-cellulosicorganic material with bleaching material comprising a bleaching reagentderived from the bleaching material supplied in step (a) and so as toform a residual liquor which includes dissolved organic matter; (c),removing such residual liquor containing soluble reaction products fromthe partially-bleached fibrous material; ((1) admixing with thispartiallybleached fibrous material additional bleach liquor including anoxidizing bleaching reagent and thereafter treating the fibers so as toconvert them into bleached chemical pulp, the total amount of availablebleaching material supplied to the fibrous material in the process beingless than would be sufficient to convert the unbleached pulp intofibre-bearing material,

high white bleached chemical pulp in a sin gle-stage bleachingtreatment. 19. The multiple-stage process of bleaching chemical woodpulp, which comprises the following steps: (a) mixing the unbleachedfibrous material with bleach liquor including an oxidizing bleachingmaterial; (6) treating the mixture so as to react aportion of thenon-cellulosic organic material with bleaching material comprising ableaching reagent derived from the bleaching material supplied in step(a) and so as to form a residual liquor which includes dissolved organicmatter; (c) removing such residual liquor containing soluble reactionproducts from the partially-bleached fibrous material; (d) admixing withthis partially bleached fibrous material additional bleach liquorincluding an oxidizing bleached reagent and thereafter treating thefibers so as to convert them into bleached chemical pulp, at least onehalf of the total amount of available bleaching material supplied to thefibrous material being supplied to the fibrous material prior to step 020. The process according to claim 19, in which nearly all of thebleaching material supplied prior to step c is exhausted prior to stepd. t

21. The process according to claim 14, in which a mixture includingfibrous material and available bleaching material in step d is heated toa moderate temperature.

22. The process according to claim 14, in

which a bleach liquor mentioned in step d as an alkaline liquor.

23. The multiple-stage 'process of bleaching chemical wood pulp, whichcomprises the folloyving steps: (a) mixing the unbleached fibrousmaterial with bleach liquor including an oxidizing bleach material; (b)treating the mixture so as to reacta portion of the non-cellulosicorganic material with bleaching material comprising a bleaching reagentderived from the bleaching material supplied in step (a) and so as toform a residual liquor which includes dissolved organic matter; (0)removing such residual liquor containing soluble reaction products fromthe partially bleached fibrous material; (d) admixing with thispartially-bleached fibrous material additional bleach liquor includingan oxidizing bleaching reagent and thereafter treatingthe fibers so asto convert them into bleached chemical pulp, one of said mixtures beingtreated with bleach liquor at low consistency.

24. The process according to claim 14,.

in which the partially bleached fibrous ma- -ing bleach material in suchamount and under such conditions as are adapted to effeet at leastone-half but less than all of the desired total bleaching removingsoluble material from resulting fibrous material, and subsequentlysubjecting such resulting partially bleached fibrous material to anoxidizing treatment adapted to complete the bleaching.

26. The method of bleaching chemical pulp from wood which comprisessubjecting the pulp to the action of a liquor containing an oxidizinbleaching reagent in amount less than su cient to complete the bleachingtreatment in a single stage and in amount such that soluble oxidizablecompounds are present, removing soluble oxidizable compounds from thepulp, and subsequently subjecting the resulting pulp, withoutintermediate cooking at temperatures above 100 C., to a bleachingtreatment with a solution containing an oxidizing bleaching agent.

In testimony whereof we aflix our signatures.

LINN BRADLEY. EDWARD P. MCKEEFE.

. CERTIFICATE OF CORRECTION.

Patent Nb. 1,768,822. Granted July 1,- 1930, to

LINN BRADLEY ET AL.

It is hereby certified that error appears in the printed speciticationolthe above numbered patent requiring correction as follows: Page 8, lines53 and 54, claim 1, for the word "compound" read compounds; page 9, linebl, claim ll, for "substantially" read subsequently; page 10, line 28,claim 19, for "bleached" read bleaching, and line 45, claim 22, for "as"read is; and that the said Letters Patent should be read with thesecorrections therein that the same may conform to the record oi the easein the Patent @ilice.

Signed and sealed this 21st day of October, A. 0. 1930,

M; J. Moore, 7

(Seal) Acting Commissioner of Patents.

